Put a simple thermometer inside a lower cabinet where your pipes run. In cold conditions after driving a distance, stop and check that you're not anywhere close to freezing.
Wind chill will remove a lot of heat while underway unless the furnace is running. There is a safety tradeoff of running LP vs. not doing so. FWIW leave lower cabinet doors ajar if you can so that heated air can actually get in to where the pipes are. Check that thermometer on occasion so you can appropriately set the thermostat. Thermostat settings even in the low 60s will burn an awful lot of LP -- or I sure did once on a 350 mile trip at 15 above.
Shower mixer valves on exterior walls are especially vulnerable to destructive freezing. I don't think the furnace will save you there if it's especially cold outside.